#op- is a prefix indicating that the word it is attached to is the target of a hacktivist attack by an individual or group under the mantle of "Anonymous." The word it is attached to could also be the entity avenged by the hacker attacks.

Anonymous C: "Why can't I get my files from MegaUpload anymore?"
Anonymous D: "Dude, have you had your head in the cloud. Don't you know the feds took down the site?"
Anonymous C: "Man,*&%$ that! #OpMegaUpload! See how the feds like their own medicine.